Welcome to the DIS! Let me say that this is probably the best place to go for planning tips and info. I practically lived on this site at night! My husband thought I was nuts until our trip last Sept with our DS (4). It was about as perfect as it could be thanks to all of the wonderful info I rec'd from people here on these boards. There is nothing like taking your child to WDW on their first trip!
I highly recommend staying onsite. If you are only going to be at Disney for 3 or four days, I would truly suggest trying to put off Sea World and the water parks for another trip. If you do go to a water park, Typhoon Lagoon is a great choice with a child. Just make sure you check any scheduled refurbishments/closures for your trip dates (allearsnet.com has that info, or check with Disney) You will be so busy with the parks! Trust everyone's recommendations to go back to the hotel for a break. As I mentioned, my DS was 4 and he really benefitted from the nap (so did DH and I). Trust everyone who recommends you rent a stroller for your child. We didn't and by midday on Day 2 my son was CRANKY! I immediately got a stroller, and didn't have one problem after that point. With a 4yo, we found it much easier to rent at the parks rather than drag one on the bus. A 4yo doesn't need a special padded stroller, and they are old enough to walk from the front of the park to a bus at the end of the day (if they rode instead of walked all day) It was our first trip with a child, and I was really impressed with the ease of stroller rental. I was nervous that it would be a pain to park for rides/shows and then pick up, but it was really easy. We had a neon green mister fan that we tied onto the handle and laid on top of the canopy. It made spotting our stroller so easy.
I highly recommend planning 2 days at MK and a visit to MGM. MGM should really only take you 1/2 a day, so try to get in AK or a visit to EPCOT. I can go on and on about what to see and do in the parks. Just private message me and I'll be happy to give you recommendations for a 4yo. In general, you might want to go to MK in the morning, have lunch in the park and head back to your resort about 2pm. Take a nice 2 hour break or so. We swam in the pool or napped (or both). I journaled about what we did and read while my son slept. It was some great "mommy time" that I don't get at home. We headed back to the park and had dinner and hit some more attractions. We were well rested for the fireworks shows at the end of the night. We tried to go to the park having the extra magic hours on the nights they were available. We went in mid-Sept when crowds were low (might be the same for you in Feb) so we got a lot out of EMH nights. It was like having a private party in the parks. All of the rides were practically walk on. One of my life's greatest memories will be the EMH night at MK when my DH was feeling sick and just my DS and I went back. We took a really long nap that day and ended up staying at the park until it closed at 1am. We went on so many rides, got an entire boat on Splash Mtn to ourselves, and were the last two people to say goodnight to Mickey and Minnie in front of the castle (I can't believe I forgot my camera - bad mom!) It was truly magical. We planned the day we did MGM to go to Epcot for their EMH night. We did MGM for the Playhouse Disney show, Beauty and the Beast, Voyage of the Little Mermaid, etc. an then after dinner and the Fantasmic Show we took the bus straight over to EPCOT and went on Test Track, Soarin', Mission Space, boat ride in Mexico and Norway, etc. (That is a great way to get in EPCOT. Most kids at 4 aren't that interested in World Showcase, and it might be better for a later trip). There is princess dining in Norway, so you might want to try and fit that in. If you can't get in to the Cinderella breakfast at MK, we did the dinner and met Cinderella on her throne in the bottom of the castle. That was a great alternative.
Something to consider if you want to see more than one resort is to eat dinner at one of the deluxe resorts. We ate at AKL and then walked out the back of the hotel to look at the animals on the Savannah. We ate at Kona Cafe at the POLY and then walked out onto the beach to view the electrical water pageant on the lake. There are so many things that you can do to maximize your vacation.
Sorry to ramble on and on... Hope this is helpful. Feel free to private message me with any questions! Have a fantastic trip!